Efficient processing of pre-mixed rubber compositions to shape and fabricate various components for articles of manufacture and to fabricate such articles themselves is important.
Various pre-mixed rubber compositions may be relatively difficult to process, such as for example by extrusion and calendering operations, if they are relatively stiff, or of a relatively high viscosity, at about room temperature, or about 23° C.
In particular, polymers and elastomers having relatively high softening point (Tm) and high glass transition point (Tg) polymers and rubbers can themselves be relatively stiff and difficult to process at room temperatures and various pre-mixed rubber compositions which contain significant amounts of such polymers and/or elastomers may consequently to relatively difficult to process at room temperature.
Representative examples of such polymers are, for example, trans 1,4-polybutadiene polymer, 3,4-polyisoprene elastomer, isoprene/butadiene copolymers having a high trans 1,4-polybutadiene component, styrene/butadiene copolymer elastomers with high styrene contents, styrene/butadiene copolymer elastomers with high trans 1,4-polybutadiene component, polystyrene and natural cis 1,4-polyisoprene rubber.
For example, trans 1,4-polybutadiene polymers are sometimes used in various rubber compositions such as, for example, various tire components.
For example, a trans 1,4-polybutadiene polymer typically has a relatively high melting point (Tm) above normal room temperature is usually considered to be at least 30° C. and generally in a range of from about 35° C. to about 45° C. The trans 1,4-polybutadiene polymer is relatively solid at room temperature (e.g. 23° C.).
The trans 1,4-polybutadiene polymer also has a relatively high crystallinity content and tends to be a thermoplastic polymer which contains numerous carbon-to-carbon double bonds in its polymeric backbone and can thereby be blended with and sulfur cured with various diene-based elastomers. It tends to become elastomeric when sulfur cured alone or in combination with diene-based elastomers.
Due to its relatively high melting point as well as its thermoplastic nature, the trans 1,4-polybutadiene is normally relatively difficult to process (e.g. mix) in ordinary internal rubber mixers so that it is often desirable to pre-heat the trans 1,4-polybutadiene before mixing it with other diene-based elastomers in the preparation of various rubber compositions for use in the preparation of components for fabricating various articles of manufacture such as, for example, tires.
In practice, the trans 1,4-polybutadiene polymer is often simply pre-heated to a temperature to, for example, at least about 40° C., usually at least about 45° C., to render it more readily processable prior to mixing it with other diene-based elastomers. Such preheating may be accomplished by placing a bale of trans 1,4-polybutadiene in a “hot house” storage room over a period of time of several days. However, during the relatively slow “hot house” heating typically causes the trans 1,4-polybutadiene polymer to undergo oxidative gelation which, in turn, can cause the resulting trans 1,4-polybutadiene to be difficult to process even though it is in a pre-heated state.
Alternatively, the trans 1,4-polybutadiene polymer may be processed in its polymerizate cement state by may be pre-blended the polymerizate with a synthetic cis 1,4-polybutadiene elastomer polymerizate cement and the resulting blend recovered to yield a more processable trans 1,4-polybutadiene polymer, in a form of a masterbatch with the cis 1,4-polyisoprene elastomer, to thereby avoid the slow “hot house” pre-heating treatment of the trans 1,4-polybutadiene polymer. For example, see U.S. Pat. No. 5,753,761.
Alternatively, by rapidly pre-heating the trans 1,4-polybutadiene polymer, particularly when containing an aromatic a rubber processing oil, to a temperature of at least 40° C., and alternately at least about 45° C., via application of radio frequency (electromagnetic) treatment at a frequency in a range of about 2 MHz to about 80 MHz, the trans 1,4-polybutadiene may be rendered processable without an unnecessarily excessive attendant oxidative cross-linking and gelation which may be experienced by the more time consuming “hot house” pre-heating of the trans 1,4-polybutadiene polymer over a period of several days. For example, see U.S. Pat. No. 5,854,351.
In practice, however, in a pre-mixed rubber composition which contains a significant presence of a high melting point (Tm) and/or high glass transition temperature (Tg) polymer and/or elastomer, such as for example, the trans 1,4-polybutadiene, such pre-mixed rubber composition itself may thereby also significantly stiff, or of a significantly high viscosity to render it relatively difficult to process in conventional rubber processing equipment for shaping and fabricating (for example by rubber extruder/die apparatus and textile fabric rubber encapsulating calendering equipment). Accordingly, often only a minimal amount (e.g. perhaps less than about 10 phr) of the trans 1,4-polybutadiene polymer can normally satisfactorily be used in such rubber compositions without incurring processing difficulties in shaping and calendering such relatively stiff (e.g. high viscosity) rubber composition for shaping and fabricating various rubber components for various articles of manufacture such as, for example, tires.
It is therefore desired to provide a process of shaping and fabrication of various components of an article of manufacture of a pre-mixed rubber composition which contains a significant content of polymer and/or elastomer having a Tm or Tg above 0° C. and particularly above 30° C. (particularly and clearly above a room temperature of 23° C.), and particularly including a trans 1,4-polybutadiene polymer and particularly a stiff, or relatively high viscosity rubber composition composed of diene-based elastomer(s) which contains at least 20, and preferably from about 30 to about 80, phr of the high Tm or high Tg polymer or elastomer such as, for example a trans 1,4-polybutadiene polymer.
Thus, it is considered herein that, in general, a problem exists for rubber compositions which contain an appreciable content of the relatively high Tm or high Tg polymer or elastomer, such as for example, a trans 1,4-polybutadiene (e.g. greater that 10 phr and particularly greater than 20 phr, depending somewhat upon the nature and composition of the rubber composition itself), thereby rendering the rubber composition relatively stiff at room temperature (e.g. 25° C.) and difficult to process with conventional rubber processing equipment (e.g. shaping the rubber composition via extrusion of the uncured rubber composition through various shaping dies or calendering onto a textile fabric) to form a component of an article of manufacture such as, for example, a tire. Therefore, it is considered herein that the use of such rubber compositions to shape and/or fabricate various components for articles of manufacture, such as for example tires, which contain a significant content of high Tm or high Tg polymer is significantly limited.
Accordingly, it is an aspect of this invention to provide a process of shaping and/or fabrication of a component of article of manufacture, and the fabrication of the article itself, for example a tire, and thereby providing such component and such article of manufacture, by pre-treating at least one component thereof as a silica-rich diene rubber based rubber composition which contains a sulfur curative and which contains at least 20, alternately from about 30 to about 80, phr of relatively high Tm or high Tg polymer or elastomer, such as for example, trans 1,4-polybutadiene and contains less than about 10 phr, preferably a maximum of about 6 phr, of carbon black by directed high frequency energy heating (e.g. radio frequency or by microwave heating) thereof to a temperature of at least about 40° C., and preferably at least about 45° C., to a maximum of about 90° C. and preferably a maximum of about 75° C., prior the physical processing of the rubber composition to form a rubber component for fabrication of an article of manufacture which contains such component. Preferably such pre-heated temperature range is maintained subsequent to said pre-heating and until the article is fabricated (e.g. the temperature of the rubber composition is preferably not allowed to drop below 40° C., preferably not drop below 45° C.).
In the description of this invention, the term “phr” refers to parts by weight of an ingredient per 100 parts by weight of both the diene-based elastomer(s), as well as a high Tm or high Tg polymer or elastomer, such as for example, a trans 1,4-polybutadiene polymer when it is used, to be included in a rubber composition.
The softening point (Tm) and glass transition temperature (Tg) of a polymer or elastomer may be determined by differential scanning calorimeter means (DSC), a method of determining a Tm or Tg of a polymer well known to those having skill in such art, at a heating rate of 10° C. per minute (ASTM D3418-99)
The terms “elastomer” and “rubber” may be used interchangeably unless otherwise indicated. The terms “cured” and “vulcanized” where used, may be used interchangeably, unless otherwise indicated.